A drilling installation constituted by drill strings enabling information to be transmitted between the bottom of a borehole being drilled and the surface of the ground is already described in French patent 2 777 594 in the name of the Applicant. As explained in that document, while a borehole is being drilled, it is very important to be able to transmit to the surface information that is collected by sensors mounted in the vicinity of the drilling tool, which tool is fixed to the bottom end of the drill string.
One of the solutions proposed in the above-mentioned French patent is described below with reference to accompanying FIG. 1.
FIG. 1 shows a drill string made up of a rod 10a constituting the bottom rod, a rod 10b constituting the top rod, and intermediate rods. A drilling tool 12 is fixed to the bottom end of the rod 10a and has measurement sensors 14. The inside face 16 of the rod 10 is coated in a layer of insulating material 18 over the entire length of the drill string. Furthermore, the drill string is disposed inside a borehole 20 that is being drilled, which borehole is filled with drilling mud 22 that conducts electricity. To enable information to be transmitted, there is a first induction coil 24 disposed close to the bottom end of the rod 10a and connected to the measurement sensor 14. There is also an inductive coupler coil 26 mounted inside the top rod 10b and connected to conductors such as 28 for transmitting electrical signals picked up by the coil 26 to a processor device.
Because of the presence of the insulating layer 16, a closed current loop is established constituted firstly through the drilling mud 30 filling the inside of the rod 10 and secondly through the assembly constituted by the wall of the rod 10 and by the mud 32 outside the rod 10.
Using the bottom coil 24, alternating current (AC) representing information is induced in the current loop, with this AC being picked up by the receiver coil 26.
Such a installation for transmitting information via a drill string gives satisfactory results in some situations. Nevertheless, it will be understood that information is transmitted via two electromagnetic couplings corresponding to the two coils or toruses which are disposed respectively close to the top end of the drill string and close to its bottom end carrying the tool. These two couplings, constituting current transformers, present the particular characteristic of comprising a very large number of turns wound on toruses. In contrast, the other element of the coupling is constituted by a single turn and consists in the single current loop established essentially via the mud contained in the drill string. Electromagnetic coupling of that type is not optimal and its efficiency or effectiveness can turn out to be insufficient, particularly when the electrical conductivity of the mud is low.